1. Technical Field
The present invention relates to adhesive applicators and bookbinding apparatuses employing the adhesive applicators, wherein sheets sequentially conveyed out from an image forming device or other printing machine are registered into a set by aligning the sheets and stacking them into a bundle, and glue or other adhesive is applied to an edge of the sheet bundle; more particularly the invention relates to improvements in temperature control when a solid hot-melt adhesive is charged into a tub-shaped container and melted at a predetermined temperature by a heater with which the container is equipped.
2. Description of the Related Art
Generally, this kind of bookbinding apparatus is widely used as a terminal device of an image forming apparatus such as a printer or printing machine, to stack and align printed sheets in page order to form a bundle, to apply adhesive to an edge thereof and then to bind that sheet bundle to a cover sheet. Adhesive application devices built into such bookbinding apparatuses use a container that holds adhesive such as glue and an applicator roller provided inside the container to apply liquefied adhesive to a side edge of the sheet bundle. A heater is built into the container to melt solid adhesive filled into the container and to maintain the liquefied adhesive at a temperature at which the viscosity of the adhesive is appropriate for adhesion.
This method of thus supplying the solid adhesive into the device interior and then heating the adhesive to melt it is characterized by ease of handling the adhesive. However, close attention must be paid to controlling the temperature of the adhesive after it has melted in the container. For example, the melting point of ordinarily employed solid adhesives is on the order of from 60° C. to 80° C., and onto material such as sheets to be glued, the adhesive must be kept at a temperature between 140° C. and 150° C. Should the adhesive temperature happen to be lower than its optimum temperature, clumps in the form of solids that have not melted completely may be included in the container, or strongly viscid (high-viscosity) adhesive may be applied to the sheet bundle. This situation can lead to trouble such as leaves missing from a glued booklet, owing to the adhesive not having permeated the sheet bundle between its pages.
Also, if the temperature of the adhesive is higher than the optimum temperature mentioned above, viscosity will become lower (or weaker) and this will cause a problem of droplets of adhesive being splattered in the process of applying the adhesive to a sheet bundle. This can cause the cover sheet to become soiled or stained. Concurrent with these problems, the melting parameters after a hot-melt adhesive is charged into a container differ depending on whether the fill quantity is a large-volume or small-volume. The melted adhesive in the container re-solidifies when the apparatus is in disuse for an extended period. Moreover, the degree of solidification is also affected by the ambient temperature. Therefore, when starting the bookbinding apparatus, it is necessary to quickly melt new, solid adhesive or adhesive that has re-solidified, and to maintain the adhesive at a predetermined temperature.
In conventional solid adhesive temperature control, a warming mode, such as that disclosed in Japanese Unexamined Pat. App. Pub. No. 2005-238526, is provided so that the liquefied adhesive in the container does not re-solidify when the apparatus is in disuse for an extended period and adhesive is not being applied to sheets. This document discloses warming modes that when adhesive is not being applied to sheet bundles in the bookbinding apparatus, or when the apparatus is idle, maintain the adhesive container at a temperature lower than the application temperature.
Pub. No. 2005-238526 also discloses providing a glue-storing container with an induction heating coil, and melting the adhesive in the container with the Joule heat from eddy currents due to the high-frequency magnetic flux generated in the coil. Also disclosed is adjusting the current supplied to the coil according to the adhesive temperature detected by a sensor (thermistor) equipped in the container. The same publication discloses providing mixing means to keep adhesive melted in the container at a uniform temperature.
As described above, when applying an adhesive inside a container to a sheet bundle in a bookbinding apparatus or similar device, employing a hot-melt adhesive that becomes solid at ordinary temperatures facilitates handling. Drawbacks with such adhesives are that when the apparatus is in non-operational or on standby, the liquefied adhesive solidifies, and that when the apparatus is started up, solidified adhesive, or freshly replenished adhesive, must in a short period of time be dissolved and brought to a temperature appropriate for its application. Liquefying (bringing to the appropriate temperature) solid adhesive is time-consuming. This causes the problem of having to wait to operate the machine until the adhesive has sufficiently melted.
To address such problems, to date it has been proposed, as disclosed in Japanese Unexamined Pat. App. Pub. No. 2005-238526 to maintain a heating means for the container in an operational state when the apparatus is idle. Specifically, current continues to energize the heating elements of a heater while the apparatus is idle, but this results in wasted energy consumption. There is also the danger of causing a fire if current is continually supplied to the heating elements while the machine is not in use. An additional drawback is that preparation time for the adhesive to melt is required when starting up the apparatus.
Furthermore, as disclosed in Japanese Unexamined Pat. App. Pub. No. 2003-010748, attempts have been made to dissolve the container contents in a short time with a high-frequency heating device. This approach, however, leads to high-cost and safety issues, because ordinary high-frequency heating devices operate at frequencies appreciably higher than the frequency at which commercial power is supplied.
Thus, within bookbinding apparatuses or like machines hot-melt adhesives—solidified adhesive when starting up a machine or when restarting an idle machine—must be dissolved in a short time, but with employing a large-capacity heating device such as a high-frequency heating apparatus having been the common practice to date, the problems for bookbinding apparatuses made compact and all purpose in office equipment have been increased size, higher cost, and higher power consumption. A concurrent problem has been that because liquefied adhesive in the container cannot be expected to circulate by convection the adhesive must be stirred; and as disclosed in Pat. App. Pub. No. 2003-010748, the adhesive must be mixed at the same time it is being melted.
A problem in this regard has been that if the viscosity of the liquefied adhesive is high, the adhesive exerts excessive load on the mixing means and its drive mechanism, which proves to be a cause of mechanical failure. In other words, if a solid adhesive is stirred too early after the adhesive has been dissolved, the viscosity load produces an overload on the drive motor. This can cause faulty operation.